The present invention relates to a process for the preparation of taxol, baccatin III and 10-desacetyl-baccatin III derivatives or other taxanes having new C9 functional groups.
Taxol is a natural product extracted from the bark of yew trees. It has been shown to have excellent antitumor activity in in vivo animal models, and recent studies have elucidated its unique mode of action, which involves abnormal polymerization of tubulin and disruption of mitosis. It is currently undergoing clinical trials against ovarian, breast and other types of cancer in the United States and France and preliminary results have confirmed it as a most promising chemo-therapeutic agent. The structure of taxol and the numbering system conventionally used is shown below; this numbering system is also applicable to compounds used in the process of the present invention. 
In Colin U.S. Pat. No. 4,814,470, it was reported that a taxol derivative, commonly referred to as taxotere, has an activity significantly greater than taxol. Taxotere has the following structure: 
The tetracyclic core of taxol and taxotere bear a C9 keto substituent which, if modified, would lead to the preparation of a series of taxol analogs having improved water solubility. To date, however, the selective manipulation of the C9 keto group has presented a formidable problem.
Among the objects of the present invention, therefore, is the provision of a process for selectively manipulating the C9 keto substituent of baccatin III, 10-deactylbaccatin III and other taxanes; and the provision of such a process which is relatively straightforward.
Briefly, therefore, the present invention is directed to a process for the preparation of analogs or derivatives of taxol, baccatin III, 10-desacetyl baccatin III or other taxanes in which the C9 keto substituent is reduced to the corresponding hydroxy group. Optionally, the C9 hydroxy substituent may thereafter be selectively replaced by another functional group and/or other substituents of the taxane may be replaced by other functional groups to yield a taxane having the formula: 
wherein
R1 is hydrogen, hydroxy, protected hydroxy or together with R14 forms a carbonate;
R2 is hydrogen, hydroxy, xe2x80x94OCOR31, or together with R2a forms an oxo;
R2a is hydrogen or together with R2 forms an oxo;
R4 is hydrogen, together with R4a forms an oxo, oxirane or methylene, or together with R5a and the carbon atoms to which they are attached form an oxetane ring;
R4a is hydrogen, alkyl, alkenyl, alkynyl, aryl, heteroaryl, cyano, hydroxy, xe2x80x94OCOR30, or together with R4 forms an oxo, oxirane or methylene;
R5 is hydrogen or together with R5a forms an oxo;
R5a is hydrogen, hydroxy, protected hydroxy, acyloxy, together with R5 forms an oxo, or together with R4 and the carbon atoms to which they are attached form an oxetane ring;
R6 is hydrogen, alkyl, alkenyl, alkynyl, aryl, or heteroaryl, hydroxy, protected hydroxy or together with R6a forms an oxo;
R6a is hydrogen, alkyl, alkenyl, alkynyl, aryl, or heteroaryl, hydroxy, protected hydroxy or together with R6 forms an oxo;
R7 is hydrogen or together with R7a forms an oxo;
R7a is hydrogen, halogen, protected hydroxy, xe2x80x94OR28, or together with R7 forms an oxo;
R9 is hydrogen;
R9a is hydrogen, hydroxy, protected hydroxy, or acyloxy;
R10 is hydrogen or together with R10a forms an oxo;
R10a is hydrogen, xe2x80x94OCOR29, hydroxy or protected hydroxy, or together with R10 forms an oxo;
R13 is hydroxy, protected hydroxy or 
R14 is hydrogen, alkyl, alkenyl, alkynyl, aryl, or heteroaryl, hydroxy, protected hydroxy or together with R1 forms a carbonate;
R14a is hydrogen, alkyl, alkenyl, alkynyl, aryl, or heteroaryl;
R29, is hydrogen, acyl, or hydroxy protecting group;
R29, R30 and R31 are independently hydrogen, alkyl, alkenyl, alkynyl, monocyclic aryl or monocyclic heteroaryl;
X1 is xe2x80x94OX6, xe2x80x94SX7, or xe2x80x94NX8X9;
X2 is hydrogen, alkyl, alkenyl, alkynyl, aryl, or heteroaryl;
X3 and X4 are independently hydrogen, alkyl, alkenyl, alkynyl, aryl, or heteroaryl;
X5 is xe2x80x94COX10, xe2x80x94COOX10, xe2x80x94COSX10, xe2x80x94CONX8X10, or xe2x80x94SO2X11;
X6 is hydrogen, alkyl, alkenyl, alkynyl, aryl, heteroaryl, or hydroxy protecting group;
X7 is alkyl, alkenyl, alkynyl, aryl, heteroaryl, or sulfhydryl protecting group;
X8 is hydrogen, alkyl, alkenyl, alkynyl, aryl, heteroaryl, or heterosubstituted alkyl, alkenyl, alkynyl, aryl or heteroaryl;
X9 is an amino protecting group;
X10 is alkyl, alkenyl, alkynyl, aryl, heteroaryl, or heterosubstituted alkyl, alkenyl alkynyl, aryl or heteroaryl;
X11 is alkyl, alkenyl, alkynyl, aryl, heteroaryl, xe2x80x94OX10, or xe2x80x94NX8X14; and
X14 is hydrogen, alkyl, alkenyl, alkynyl, aryl, or heteroaryl.
The present invention is additionally directed to a derivative of baccatin III or 10-desacetyl baccatin III having the following formula which is a key intermediate in the synthesis of a new series of tetracyclic taxanes 
wherein R2, R4a, R7a, R10a and R14 are as previously defined and R13 is hydroxy or protected hydroxy.
Other objects and features of this invention will be in part apparent and in part pointed out hereinafter.
As used herein xe2x80x9cArxe2x80x9d means aryl; xe2x80x9cPhxe2x80x9d means phenyl; xe2x80x9cAcxe2x80x9d means acetyl; xe2x80x9cEtxe2x80x9d means ethyl; xe2x80x9cRxe2x80x9d means alkyl unless otherwise defined; xe2x80x9ctBuxe2x80x9d means t-butyl; xe2x80x9cTESxe2x80x9d means triethylsilyl; xe2x80x9cTMSxe2x80x9d means trimethylsilyl; xe2x80x9cTPAPxe2x80x9d means tetrapropylammonium perruthenate; xe2x80x9cDMAPxe2x80x9d means p-dimethylamino pyridine; xe2x80x9cDMFxe2x80x9d means dimethyl-formamide; xe2x80x9cLDAxe2x80x9d means lithium diisopropylamide; xe2x80x9cLAHxe2x80x9d means lithium aluminum hydride; xe2x80x9cRed-Alxe2x80x9d means sodium bis(2-methoxyethoxy) aluminum hydride; xe2x80x9c10-DABxe2x80x9d means 10-desacetylbaccatin III; protected hydroxy means xe2x80x94OR wherein R is a hydroxy protecting group; sulfhydryl protecting groupxe2x80x9d includes, but is not limited to, hemithioacetals such as 1-ethoxyethyl and methoxymethyl, thioesters, or thiocarbonates; xe2x80x9camine protecting groupxe2x80x9d includes, but is not limited to, carbamates, for example, 2,2,2-trichloroethylcarbamate or tertbutylcarbamate; and xe2x80x9chydroxy protecting groupxe2x80x9d includes, but is not limited to, ethers such as methyl, t-butyl, benzyl, p-methoxybenzyl, p-nitrobenzyl, allyl, trityl, methoxymethyl, methoxyethoxymethyl, ethoxyethyl, tetrahydropyranyl, tetrahydrothiopyranyl, and trialkylsilyl ethers such as trimethylsilyl ether, triethylsilyl ether, dimethylarylsilyl ether, triisopropylsilyl ether and t-butyldimethylsilyl ether; esters such as benzoyl, acetyl, phenylacetyl, formyl, mono-, di-, and trihaloacetyl such as chloroacetyl, dichloroacetyl, trichloroacetyl, trifluoroacetyl; and carbonates including but not limited to alkyl carbonates having from one to six carbon atoms such as methyl, ethyl, n-propyl, isopropyl, n-butyl, t-butyl; isobutyl, and n-pentyl; alkyl carbonates having from one to six carbon atoms and substituted with one or more halogen atoms such as 2,2,2-trichloroethoxymethyl and 2,2,2-trichloroethyl; alkenyl carbonates having from two to six carbon atoms such as vinyl and allyl; cycloalkyl carbonates have from three to six carbon atoms such as cyclopropyl, cyclobutyl, cyclopentyl and cyclohexyl; and phenyl or benzyl carbonates optionally substituted on the ring with one or more C1-6 alkoxy, or nitro. Other hydroxyl, sulfhydryl and amine protecting groups may be found in xe2x80x9cProtective Groups in Organic Synthesisxe2x80x9d by T. W. Greene, John Wiley and Sons, 1981.
The alkyl groups described herein, either alone or with the various substituents defined hereinabove are preferably lower alkyl containing from one to six carbon atoms in the principal chain and up to 15 carbon atoms. They may be straight or branched chain and include methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, aryl, hexyl, and the like.
The alkenyl groups described herein, either alone or with the various substituents defined hereinabove are preferably lower alkenyl containing from two to six carbon atoms in the principal chain and up to 15 carbon atoms. They may be straight or branched chain and include ethenyl, propenyl, isopropenyl, butenyl, isobutenyl, aryl, hexenyl, and the like.
The alkynyl groups described herein, either alone or with the various substituents defined hereinabove are preferably lower alkynyl containing from two to six carbon atoms in the principal chain and up to 15 carbon atoms. They may be straight or branched chain and include ethynyl, propynyl, butynyl, isobutynyl, aryl, hexynyl, and the like.
The aryl moieties described herein, either alone or with various substituents, contain from 6 to 15 carbon atoms and include phenyl. Substituents include alkanoxy, protected hydroxy, halogen, alkyl, aryl, alkenyl, acyl, acyloxy, nitro, amino, amido, etc. Phenyl is the more preferred aryl.
The heteroaryl moieties described herein, either alone or with various substituents, contain from 5 to 15 atoms and include, furyl, thienyl, pyridyl and the like. Substituents include alkanoxy, protected hydroxy, halogen, alkyl, aryl, alkenyl, acyl, acyloxy, nitro, amino, amido, etc.
The acyloxy groups described herein contain alkyl, alkenyl, alkynyl, aryl or heteroaryl groups.
The substituents of the substituted alkyl, alkenyl, alkynyl, aryl, and heteroaryl groups and moieties described herein, may be alkyl, alkenyl, alkynyl, aryl, heteroaryl and/or may contain nitrogen, oxygen, sulfur, halogens and include, for example, lower alkoxy such as methoxy, ethoxy, butoxy, halogen such as chloro or fluoro, nitro, amino, and keto.
In accordance with the present invention, it has been discovered that the C9 keto substituent of a taxane having a C9 keto and a C7 hydroxy substuent may be selectively reduced to yield the corresponding C9 xcex2-hydroxy derivative. The reducing agent is preferably a an aluminum hydride or a borohydride such as triacetoxyborohydride, more preferably a tetraalkylborohydride or tetraalkylaluminumhydride, and most preferably, tetrabutylammoniumborohydride (Bu4NBH4).
As illustrated in Reaction Scheme 1, the reaction of baccatin III with Bu4NBH4 in methylene chloride yields 9-desoxo-9xcex2-hydroxybaccatin III 5. After the C7 hydroxy group is protected with, for example, the triethylsilyl protecting group, a suitable side chain may be attached to 7-protected-9xcex2-hydroxy derivative 6 as set forth in U.S. Pat. Nos. 4,924,011 and 4,924,012 or by reaction with a xcex2-lactam as set forth in U.S. Pat. No. 5,175,315 or copending U.S. patent application Ser. No. 07/949,107. Removal of the remaining protecting groups thus yields 9xcex2-hydroxy-desoxo taxol or other 9xcex2-hydroxy-tetracylic taxane having a C13 side chain. 
Alternatively, the C13 hydroxy group of 7-protected-9xcex2-hydroxy derivative 6 may be protected with trimethylsilyl or other protecting group which can be selectively removed relative to the C7 hydroxy protecting group as illustrated in Reaction Scheme 2, to enable further selective manipulation of the various substituents of the taxane. For example, reaction of 7,13-protected-9xcex2-hydroxy derivative 7 with KH causes the acetate group to migrate from C10 to C9 and the hydroxy group to migrate from C9 to C10, thereby yielding 10-desacetyl derivative 8. Protection of the C10 hydroxy group of 10-desacetyl derivative 8 with triethylsilyl yields derivative 9. Selective removal of the C13 hydroxy protecting group from derivative 9 yields derivative 10 to which a suitable side chain may be attached as described above. 
As shown in Reaction Scheme 3, 10-oxo derivative 11 can be provided by oxidation of 10-desacetyl derivative 8. Thereafter, the C13 hydroxy protecting group can be selectively removed followed by attachment of a side chain as described above to yield 9-acetoxy-10-oxo-taxol or other 9-acetoxy-10-oxotetracylic taxanes having a C13 side chain. Alternatively, the C9 acetate group can be selectively removed by reduction of 10-oxo derivative 11 with a reducing agent such as samarium diiodide to yield 9-desoxo-10-oxo derivative 12 from which the C13 hydroxy protecting group can be selectively removed followed by attachment of a side chain as described above to yield 9-desoxo-10-oxo-taxol or other 9-desoxo-10-oxotetracylic taxanes having a C13 side chain. 
Reaction Scheme 4 illustrates a reaction in which 10-DAB is reduced to yield pentaol 13. The C7 and C10 hydroxyl groups of pentaol 13 can then be selectively protected with the triethylsilyl or another protecting group to produce triol 14 to which a C13 side chain can be attached as described above or, alternatively, after further modification of the tetracylic substituents. 
Taxanes having C9 and/or C10 acyloxy substituents other than acetate can be prepared using 10-DAB as a starting material as illustrated in Reaction Scheme 5. Reaction of 10-DAB with triethylsilyl chloride in pyridine yields 7-protected 10-DAB 15. The C10 hydroxy substituent of 7-protected 10-DAB 15 may then be readily acylated with any standard acylating agent to yield derivative 16 having a new C10 acyloxy substituent. Selective reduction of the C9 keto substituent of derivative 16 yields 9xcex2-hydroxy derivative 17 to which a C13 side chain may be attached. Alternatively, the C10 and C9 groups can be caused to migrate as set forth in Reaction Scheme 2, above. 
9-desoxo tetracyclic taxanes having alternative C2 and/or C4 esters can be prepared using baccatin III and 10-DAB as starting materials. The C2 and/or C4 esters of baccatin III and 10-DAB can be selectively reduced to the corresponding alcohol(s) using reducing agents such as LAH or Red-Al, and new esters can thereafter be substituted using standard acylating agents such as anhydrides and acid chlorides in combination with an amine such as pyridine, triethylamine, DMAP, or diisopropyl ethyl amine. Alternatively, the C2 and/or C4 alcohols may be converted to new C2 and/or C4 esters through formation of the corresponding alkoxide by treatment of the alcohol with a suitable base such as LDA followed by an acylating agent such as an acid chloride.
Baccatin III and 10-DAB analogs having different substituents at C2 and/or C4 can be prepared as set forth in Reaction Schemes 6-10. To simplify the description, 10-DAB is used as the starting material. It should be understood, however, that baccatin III derivatives or analogs may be produced using the same series of reactions (except for the protection of the C10 hydroxy group) by simply replacing 10-DAB with baccatin III as the starting material. Derivatives of the baccatin III and 10-DAB analogs having different substituents at C9 and at least one other position, for instance C1, C2, C4, C7, C10 and C13, can then be prepared by carrying out any of the other reactions described herein and any others which are within the level of skill in the art.
In Reaction Scheme 6, protected 10-DAB 3 is converted to the triol 18 with lithium aluminum hydride. Triol 18 is then converted to the corresponding C4 ester using Cl2CO in pyridine followed by a nucleophilic agent (e.g., Grignard reagents or alkyllithium reagents). 
Deprotonation of triol 18 with LDA followed by introduction of an acid chloride selectively gives the C4 ester. For example, when acetyl chloride was used, triol 18 was converted to 1,2 diol 4 as set forth in Reaction Scheme 7.
Triol 18 can also readily be converted to the 1,2 carbonate 19. Acetylation of carbonate 19 under vigorous standard conditions provides carbonate 21 as described in Reaction Scheme 8; addition of alkyllithiums or Grignard reagents to carbonate 19 provides the C2 ester having a free hydroxyl group at C4 as set forth in Reaction Scheme 6. 
As set forth in Reaction Scheme 9, other C4 substituents can be provided by reacting carbonate 19 with an acid chloride and a tertiary amine to yield carbonate 22 which is then reacted with alkyllithiums or Grignard reagents to provide 10-DAB derivatives having new substituents at C2. 
Alternatively, baccatin III may be used as a starting material and reacted as shown in Reaction Scheme 10. After being protected at C7 and C13, baccatin III is reduced with LAH to produce 1,2,4,10 tetraol 24. Tetraol 24 is converted to carbonate 25 using Cl2CO and pyridine, and carbonate 25 is acylated at C10 with an acid chloride and pyridine to produce carbonate 26 (as shown) or with acetic anhydride and pyridine (not shown). Acetylation of carbonate 26 under vigorous standard conditions provides carbonate 27 which is then reacted with alkyl lithiums to provide the baccatin III derivatives having new substituents at C2 and C10. 
10-desacetoxy derivatives of baccatin III and 10-desoxy derivatives of 10-DAB may be prepared by reacting baccatin III or 10-DAB (or their derivatives) with samarium diiodide. Reaction between the tetracyclic taxane having a C10 leaving group and samarium diiodide may be carried out at 0xc2x0 C. in a solvent such as tetrahydrofuran. Advantageously, the samarium diiodide selectively abstracts the C10 leaving group; C13 side chains and other substituents on the tetracyclic nucleus remain undisturbed. Thereafter, the C9 keto substituent may be reduced to provide the corresponding 9-desoxo-9xcex2-hydroxy-10-desacetyoxy or 10-desoxy derivatives as otherwise described herein.
C7 dihydro and other C7 substituted taxanes can be prepared as set forth in Reaction Schemes 11, 12 and 12a. 
As shown in Reaction Scheme 12, Baccatin III may be converted into 7-fluoro baccatin III by treatment with PAR at room temperature in THF solution. Other baccatin derivatives with a free C7 hydroxyl group behave similarly. Alternatively, 7-chloro baccatin III can be prepared by treatment of baccatin III with methane sulfonyl chloride and triethylamine in methylene chloride solution containing an excess of triethylamine hydrochloride.
Taxanes having C7 acyloxy substituents can be prepared as set forth in Reaction Scheme 12a, 7,13-protected 10-oxo-derivative 11 is converted to its corresponding C13 alkoxide by selectively removing the C13 protecting group and replacing it with a metal such as lithium. The alkoxide is then reacted with a xcex2-lactam or other side chain precursor. Subsequent hydrolysis of the C7 protecting groups causes a migration of the C7 hydroxy substituent to C10, migration of the C10 oxo substituent to C9, and migration of the C9 acyloxy substituent to C7.
A wide variety of tricyclic taxanes are naturally occurring, and through manipulations analogous to those described herein, an appropriate side chain can be attached to the C13 oxygen of these substances. Alternatively, as shown in Reaction Scheme 13, 7-O-triethylsilyl baccatin III can be converted to a tricyclic taxane through the action of trimethyloxonium tetrafluoroborate in methylene chloride solution. The product diol then reacts with lead tetraacetate to provide the corresponding C4 ketone. 
Recently a hydroxylated taxane (14-hydroxy-10-deacetylbaccatin III) has been discovered in an extract of yew needles (CandEN, p 36-37, Apr. 12, 1993). Derivatives of this hydroxylated taxane having the various C2, C4, etc. functional groups described above may also be prepared by using this hydroxylated taxane. In addition, the C14 hydroxy group together with the C1 hydroxy group of 10-DAB can be converted to a 1,2-carbonate as described in CandEN or it may be converted to a variety of esters or other functional groups as otherwise described herein in connection with the C2, C4, C9 and C10 substituents.
The following examples are provided to more fully illustrate the invention.